Yahoo! Mail vs Postbox

With this Yahoo! Mail feature users can open any e-mail in a new tab and get back to the mail list without having to close the e-mail that the user is currently reading.

Similar to Mail app bundled in Windows 10, Yahoo! Mail allows users to set images from Flickr as the background image for the app.

Stationary is an add-on that's only available in Yahoo! Mail. It allows users to customize and enhance the look of the email. Things like image borders, letter style etc. can be changed before sending the email to intended recipients.

Reminding the user that they forgot to attach the file if they mentioned one in the body of the email is one of the most useful features of any email client and so many don't include this. Postbox is pre-configured to look for certain keywords (attached, CV, spreadsheet, pdf etc) and you can specify your own keywords in addition.

Has the ability to block tracking gifs commonly used for read receipts.

All messages in one place. But you can select to show only messages from each account either.

Quick bar allows you to add a variety of content to your message. It will let you reuse URLS in your Sent folder, and autocompletes against canned responses, signatures, and topics.